Amplitude regulator



Aug. 12; 1958 l w. DONNER 2,847,574

AMPLITUDE REGULATOR Filed Oct. 16, 1956 2 Sheets-Sheet 1 Retording M I$ R F EXCI/Qf Scarce 729 Vacuum Pump lia.

INl/ENTOR. WALTER DONNEJ? BY H/S fiTTORNEY6. HHRR/b) Knee-H, P057251? 8: HHRR/S United States AMPLITUDE REGULATOR Application October 16, 1956, Serial No. 616,178

9 Claims. (Cl. 250-36) This invention relates to circuitry for precisely controlling the amplitude of the output of a radio frequency oscillator and, in particular, relates to amplitude regulator circuitry for use with the radio frequency excitation source of a radio frequency mass spectrometer tube.

As general background, mass spectrometers of this general type include: an ion source; ion analyzer means for selectively varying the kinetic energies of the ions emitted by the source in accordance with their respective masses, or more accurately, in accordance with their respective mass-to-charge ratios; and selective ion collector means. The analyzer provides selected ions of a selected mass-to-charge ratio with an optimum kinetic energy level while providing nonselected ionsof other mass-to-charge ratios with other kinetic energy levels differing from the optimum. The ion collector separates the selected ions from the nonselected ions on the basis of the energy level differences produced by the analyzer means, the ion collector providing a collecting electrode, or collecting electrode means for collecting the selected ions. The optimum kinetic energy level with which the selected or preferred ions are provided may be either a maximum or a minimum, the ion collector shown in the present invention being used with an instrument which maximizes the kinetic energy of the preferred ions.

For successful operation of such equipment, it is required that the amplitude of the radio frequency source which excites the .analyzer means for carrying the kinetic energies of the ions have a long term regulation in the order of 0.01%. The load presented by the mass specrtrometer tube to the excitation source is substantially pure capacitance, and, in a typical tube, 125 voltspeak isapplied across the load. Furthermore, in. the operation of a mass spectrometer tube, the frequency of the excitation may be varied, such as by a mechanical scanning device, and the amplitude'regulation must be maintained througout the frequency range.

Accordingly, it is an object of the invention to provide an amplitude regulator circuit for use with a radio frequency oscillator for maintaining the output of the oscillator constant within very close limits. A further object of the invention is to provide such a circuit for use with substantially purely capacitive loads or one in which the circuit has no loading effect upon the oscillator. A

further object of the invention is to provide such a circui't which will function while the frequency of the oscillato'routput is varied over a wide range.

Itis a further object of the invention to provide a circuit for closely controlling the amplitude of a radio frequency oscillator as a function of the frequency of the oscillator output.

It is an object of the invention to provide an excitation source for a radio frequency massspectrometer tube hav .ing thev amplitude thereof maintained within very close limits for both short and very long periods. Another object of the invention is to provide circuitry for coupling the output of such a source to a spectrometer tube so 2,847,574 Patented Aug, 12, 1958 plitude detector circuit for use in regulatnig the amplitude of such equipment to as low as 0.01% or less. A further object of the invention is to provide such a detector circuit which contributes signal gain to the over-all circuitry. A further object of the invention is to provide such a detector circuit utilizing an electrometer tube having very low input capacity and substantially zero grid current.

The invention also comprises novel details of construction and novel combinations and arrangements of parts, which will more fully appear in the course of the following description. The drawings merely show and the description merely describes the preferredembodirnents of the present invention which are given by way of illustration or example.

In the drawings:

Fig. 1 is a semi-diagrammatic, longitudinal sectional view of a radio frequency mass spectrometer tube incorpora-ting the amplitude regulated excitation source of the invention;

Fig. 2 is :a block diagram showing a preferred embodiment of the amplitude regulator of the invention;

Fig. 3 is a schematic diagram of the embodiment of Fig. 2;

Fig. 4 is .a block diagram showing an alternative embodiment of the invention;

Fig. 5 is a block diagram showing another alternative embodiment of the invention;

Fig. 6 is a diagram of a frequency sensitive network suitable for use with the embodiments of Figs. 4 -and 5; and

Fig. 7 is a diagram of another frequency sensitive network suitable for use with the embodiments of Figs. 4 and 5. i

' The a-rriplituderegulator circuit of the invention isv described herein in connection with a radio frequency mass spectrometer tube which is illustrated in Fig. lof the drawings. The means for ionizing the sample, for imparting different energies to the ions of different mass, and for collecting a selected ion species are enclosed in a vacuumtight housing 11 connected to a vacuum pumping system, not shown, via a duct 12. More specifically, the sample, in the form of a 'gas or vapor, is introduced into an ion chamber 14 at a suitable pressure through an inlet tube 13. A filament 15 powered by a transformer 15a provides an electron stream 16 which is directed through the ion chamber 14 and collected by' a positively-charged electrode 17. A portion of the sample material is ionized under bombardment by the electron stream. Two sets of perforated electrodes, 19 to 21, and 22 to 24, connected to points of suitable potential on a power supply 32', draw ions out of the ion chamber, giving them an initial acceleration and collimating them into a narrow beam 18. The perforations in these and all subsequent electrodes may either be holes, or, prefer-ably, elongated slits. Thus, Fig. 1 may be interpreted as showing slits which are oriented perpendicularly to the plane of the drawing. In this case, however, it would be preferable to reorient the electron beam so that it too is perpendicular to the plane of the drawing.

The ion chamber 14, the electron beam forming means 15, 17 and the electrodes 19 to 24 constitute what may be termed the ion source. This is followed by a so-called analyzer means or analyzer section which comprises an array of perforated plates, 25, 25a, 2511, connected alternately to one terminal of a variable radio frequency excitation source 34 and a return point, e. g., ground, as shown. For a given accelerating potential at the ion source, and any given frequency and amplitude of the radio frequency excitation, only particles of a particular mass, or, strictly speaking, of a particular mass-tocharge ratio, will pass through the analyzer plates in resonance with the radio frequency signal. Ions of this selected mass are accelerated in phase with the alternating voltage through each, of the successive analyzer stages and thus acquire a maximum kenetic energy, whereas particles of different mass, higher or lower, fall out of phase and either do not emerge from the analyzer section or, at best, emerge with energies less than that of the preferred particle.

The tube includes an ion collector comprising two parallel electrodes or plates 26 and 27 inclined at an angle of substantially 45 to the entrant ion beam 18. The base plate 26 is at a low potential, for example, ground, and may be connected to the last analyzer plate 251: as shown. The plate 27 is at a relatively high potential, e. g., about one-half the ion beam energy in electron volts, or about 500 volts. Thus, there is provided between the plates 26 and 27 a substantially uniform electrostatic field having a plane of minimum field potential at the base plate 26. The ions from the analyzer section enter the space between the plates 26 and 27 through a relatively narrow entrance slit 28 in the plate 26, each ion traversing a parabolic trajectory and returning to the base plate 26, where ions of the predetermined mass-to-charge ratio pass through an ion-selective exit slit 29 in the plate 26 positioned at the point of maximum range. Particles traversing the slit 29 impinge on a collecting electrode or electrode means 30 at or near ground potential. Ions of lower or higher mass, and therefore of lower energy, fall short of the slit 29 and impinge on an ion intercepting means formed by the base plate 26 adjacent an edge 29a of the slit 29.

The numerals 32 and 33 represent, respectively, the paths of preferred and nonpreferred ions. An indicating and/or recording means 31, connected to the electrode 30, indicates and/or records the relative abundance of the selected ions.

In order to obtain optimum results in the use of such a spectrometer tube, it is required that the amplitude of the output of the radio frequency excitation source 34 be maintained constant within very close limits, in the embodiment described herein, typically within plus or minus 0.01 percent of the desired output. A preferred form of circuit for providing such regulated output is shown in Fig. 2, the circuit including a radio frequency oscillator 40, a coupling capacitor 41, a radio frequency amplitude detector 42 and an amplifier 43, the output of the oscillator 40 being coupled to a mass spectrometer tube 44 by the coupling capacitor 41. A radio frequency mass spectrometer tube presents a substantially pure capacitance load to the excitation source, this load being in the order of 250 micromicrofarads in the example described herein. In the operation of the radio frequency mass spectrometer tube for analytical purposes, the frequency of the excitation source is desirably variable, in order that a desired range of ion masses may be scanned. In the embodiment disclosed herein, the frequency of the oscillator 40 is continuously variable between two and six megacycles per second. Alternatively, the frequency of the source may be adjustable stepwise, for example, by the use of a stepping switch which selectively couples reactors of varying value into the oscillator tank circuit.

Loading efiects of the mass spectrometer tube on the variable frequency oscillator are eliminated by coupling the tube to the oscillator with a capacitance voltage divider in which the capacitance of the coupling capacitor 41 is many times less than the capacitance of the tube 44, the ratio of capacitances being preferably in the order of one to ten. The radio frequency amplitude detector 42 is coupled to the mass spectrometer tube 44 with asshort a lead as possible in order to prevent introduction of undesired signals and noise into the detector input and to minimize any distributed inductance which 41, the mass spectrometer tube 44 being coupled to a lead 45.

The oscillator 40 includes an amplifier tube 50 coupled to a high-Q tuned circuit 51 to function as a radio frequency oscillator, the frequency of the output of which is variable by varying one or both of the reactive elements in the tuned circuit. In this preferred form, a variable capacitor 52 in the tuned circuit 51 is driven by a motor scan 53 to vary the capacitance of the capacitor 52 over a range of at least 9 to 1 so that the frequency of the output will vary over a range of at least 3 to l. The amplitude of the output of the oscillator 40 is varied by controlling the bias voltage applied to a control grid 54 of the amplifier tube 50, the control grid 54 preferably being the screen grid of a tetrode or pentode amplifier tube. While it is preferred to use the oscillator circuit described and illustrated herein, which may be termed a Lampkin oscillator, it is understood that other oscillator circuits with various types of tuned circuits may be utilized.

The radio frequency amplitude detector 42 produces an output voltage, which can be a direct current voltage ,or a voltage varying in eifect'at a very low frequency,

which is a function of the amplitude of the radio fre quency alternating signal applied to the mass spectrometer tube, this direct current or low frequency output of the detector being applied to the control grid 54 of the tube 50 in the oscillator circuit through the amplifier 43.

The radio frequency alternating signal which is coupled to the mass spectrometer tube is also coupled to a control grid 60 of a vacuum amplifier tube 61 through a capacitor 62, the tube 61 being a tetrode of the electrometer type although, of course, a triode or a pentode may also be utilized. The tube 61 is operated as a plate detector with the control grid 60 being biased relative to a cathode 63 so that the tube conducts only during the positive peak portions of the radio frequency alternating voltage signal applied to the control grid 60. The plate-to-cathode and grid bias voltages are supplied by a circuit indicated generally by the arrow 64.which also supplies power for the heater-cathode 63 of the tube. In the embodiment of the circuit described herein, radio frequency excitation applied to the spectrometer tube is in the order of 125 volts peak and the detector tube 61 is biased to volts so that the tube conducts only at the very peak of the positive portion of the radio frequency excitation. By using such a vacuum amplifier tube in the detector circuit, loading on the oscillator by the detector is eliminated and stability with respect to temperature and time is substantially improved. Furthermore, since the tube 61 is operated as an amplifier, gain is achieved in the detector also.

It is also possible to operate'the detector tube 61 with a large positive rather than a negative grid bias. In this case, the negative peaks of the radio frequency signal alter the average current conducted by the tube, diminishing such current because of such negative peaks. In such case the grid is positive during most of the cycle, and the tube is conducting except during the negative peaks. To avoid excessive grid current a high value grid resistor is used. With this system as well as with the preferred circuitry of Fig. 3, the current through the detector tube is varied only by the effect of peak portions of the cycle of the radio frequency signal from the oscillator. v

The illustrated circuit of the detector 42 is preferably operated with the plate 64' of the tube 61 connected to ground through a load resistor 65 and a capacitor 66,

. having very low grid current, very low grid bias drift and very low input grid capacitance.- The use of such a tube in the detector circuit of the invention permits the achievement of the desired amplitude stability in the radio frequency excitation source even when the frequency of the source is being varied. In the particular embodiment of the invention described and illustrated herein, the long term stability is within 0.01 percent and the stability upon varying the frequency from 2 to 6 megacycles per second is within 0.001 percent.

The amplifier 43 serves to couple the direct current voltage output of the detector to the control grid 54 of the oscillator and may be conventional in nature. Inthe specific embodiment of the invention described herein, a two stage amplifier is utilized, having a dual triode tube 70 operated as a differential amplifier and feeding a pentode tube 71, the output of the latter being coupled to the control grid 54 of the oscillator tube 50.

In the above description of the invention, the amplitude of the output of the radio frequency oscillator hasbeen maintained constant even though the frequency of the output is varied. However, it is sometimes desirable to vary the amplitude of the radio frequency excitation as a function of the actual frequency. This may be accomplished by providing a frequency sensitive network, i. e., an impedance network having a transfer function which is a function of frequency, at the input to the radio frequency amplitude detector 42 or at the input to the mass spectrometer tube 44. If the frequency sensitive network is coupled at the input to the amplitude detector 42, the required transfer function of the network is the inverse of the desired amplitude-frequency relation. If the frequency sensitive network is inserted in the input tothe mass spectrometer tube, the required transfer function thereof corresponds to the desired amplitude-frequency relation.

Fig. 4 shows an alternative form of the circuit of Fig. 2 with a frequency sensitive network 75 connected ahead of the input to the mass spectrometer tube 44. Fig. 5 shows another alternative form of the circuit of Fig. 2 with a frequency sensitive network 76 connected ahead of the input to the radio frequency amplitude detector 42. An example of a suitable frequency sensitive network for use in Fig. 4 is shown in Fig. 6 and consists of a resistance 77 connected between an input terminal 78 and an output terminal 79, and a capacitance 80 connected between the output terminal 79 and circuit ground. An example of another frequency sensitive network, the transfer function of which is the inverse of the transfer function of the network of Fig. 6, is shown in Fig. 7, consisting of a resistance 81 connected between an input terminal 82 and an output terminal 83, and and inductance 84 connected between the output terminal 83 and circuit ground.

Although exemplary embodiments of the invention have been disclosed and discussed, it will be understood that the amplitude regulator of the invention may be applied in other ways and to other uses than mass spec trometry, and that the embodiments disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

I claim as my invention:

1. In a circuit for exciting the plates of a radio frequency mass spectrometer at constant amplitude and varying frequency, the combination of: a radio frequency oscillator having a control element for varying the amplitude of the output of said oscillator and having a tuned circuit with a variable reactance element therein; a radio frequency amplitude detector for producing an output which changes as a function of a high frequency input; a capacitor for coupling said tuned circuit to said detector and to said plates in signal transmitting relationship, the impedance of said capacitor being many times greater than the impedance of said plates; circuit means coupling said output of said detector to said control element in controlling relationship; and drive means for varying the reactance of said variable reactance element.

2. In a circuit for exciting the plates of a radio frequency mass spectrometer tube at constant amplitude, the combination of: a radio frequency oscillator having a control element for varying the amplitude of the output of said oscillator; a radio frequency detector including an electrometer vacuum tube having a control grid; circuit means for coupling said output of said oscillator to said control grid and to said plates in signal transmitting relationship, said detector including bias means for operating said tube so that said tube conducts current only during a peak portion of said radio frequency output, said detector providing an output varying as a function of said radio frequency output; and circuit means for coupling said output of said detector to said control element in controlling relationship.

3. In a circuit for exciting the plates of a radio frequency mass spectrometer tube at constant amplitude, the combination of: a radio frequency oscillator having a control element for varying the amplitude of the output of said oscillator; a radio frequency detector including an electrometer vacuum tube having a plate, a control grid and a cathode, said tube being operated as a plate detector with said plate connected to circuit ground through a plate load; circuit means for coupling said output of said oscillator to said control grid and to said plates in signal transmitting relationship, said detector including bias means for operating said tube so that said tube conducts current only during a peak portion of said radio frequency output thus providing an output which varies as a function of said radio frequency output; and circuit means for coupling said output of said detector to said control element in controlling relationship.

4. In a circuit for exciting the plates of a radio frequency mass spectrometer tube at constant amplitude and varying frequency, the combination of: a radio frequency oscillator having a control element. for varying the amplitude of the output of the said oscillator and having a tuned circuit with a variable reactance element therein; a radio frequency detector including an electrometer vacuum tube having a control grid; a capacitor for coupling said tuned circuit to said control grid and to said plates in signal transmitting relationship, the impedance of said capacitor being many times greater than the impedance of said plates, said detector including bias means for operating said tube so that said tube conducts current only during a peak portion of said radio frequency output from said tuned circuit, said detector providing an output which varies as a function of said radio frequency oscillator output; circuit means for coupling said output of said detector to said control element in controlling relationship; and means coupled to said reactance element for varying the reactance thereof.

5. In a circuit for exciting the plates of a radio frequency mass spectrometer tube at an amplitude which is a function of the frequency of excitation, the combination of: a radio frequency oscillator having a control element for varying the amplitude of the output of said oscillator and having a tuned circuit with a variable reactance element therein; a radio frequency amplitude detector for producing a low frequency output which is a function of a high frequency input; first circuit means for coupling said tuned circuit to said detector and to said plates in signal transmitting relationship, said first circuit means including a reactive network having a transfer function which varies with frequency; and second circuit means for coupling said output of said detector to said control element in controlling relationship; and drive means coupled to said variable reactance element for varying the reactance thereof.

6. In a circuit for regulating the amplitude of an alternating voltage signal, the combination of: signal generating means for generating an alternating voltage signal, said generating means including a control element for varying the amplitude of said signal; a vacuum amplifier tube having a cathode, a grid and a plate; means for applying at least a portion of said alternating voltage signal across said grid and a reference point of substantially fixed potential with respect to said cathode; bias means for biasing said grid with respect to said cathode so that current through said tube is varied only by the effect of peak portions of the cycle of said alternating voltage signal, thus generating a plate signal varying as a function of the amplitude variations in said alternating voltage signal; and circuit means coupled between said plate and said control element for varying said control element in response to variations in said plate signal in a sense to oppose said variations in amplitude of said alternating voltage signal.

7. In a circuit for regulating the amplitude of an alternating voltage signal, the combination of: signal generating means for generating an alternating voltage signal, said generating means including a control element for varying the amplitude of said signal; a vacuum electrometer tube having a directly heated cathode, a lowsecondary-emission grid and a plate, said tube being operated at electrode potentials producing substantially zero grid current; means for applying at least a portion of said alternating voltage signal between said grid and a reference point of substantially fixed potential with respect to said cathode; bias means for biasing said grid with respect to said cathode so that current through said tube is varied only by the efiect of peak portions of the cycle of said alternating voltage signal, thus generating a plate signal varying as a function of amplitude variations in said alternating voltage signal; and circuit means coupled between said plate and said control element for varying said control element in response to variations in said plate signal in a sense to oppose said variations in amplitude of said alternating voltage signal.

8. In a circuit for regulating the amplitude of an alternating voltage signal coupled to a capacitive load, the combination of: signal generating means for generating an alternating voltage signal across a tuned circuit, said generating means including a control element for varying the amplitude of said signal; a vacuum amplifier tube having a cathode, a grid and a plate; a capacitor for coupling said alternating voltage signal to said grid and to the capacitive load, the capacitor of said capacitor being many times less than the capacitance of said load; bias means for biasing said grid with respect to said cathode so that current through said tube is varied only by the eflFect of peak portion of the cycle of said alternating voltage signal, thus generating a plate signal varying as a function of the amplitude variations in said alternating voltage signal; and circuit means coupled between said plate and said control element for varying said control element in response to variations in said plate signal in a sense to oppose said variations in amplitude of said alternating voltage signal.

9. In a circuit for regulating the amplitude of an alternating voltage signal as a predetermined function of the frequency thereof, the combination of: signal generating means for generating an alternating voltage signal of varying frequency, said generating means including a control element for varying the amplitude of said signal; a vacuum amplifier tube having a cathode, a grid and a plate; means for applying at least a portion of said alternating voltage signal between said grid and a reference point of substantially fixed potential with respect to said cathode, said means including an impedance network having a transfer function the inverse of said predetermined function; bias means for biasing said grid with respect to said cathode so that said current through said tube is variedonly by the effect of peak portion of the cycle of said alternating voltage signal, thus generating a plate signal varying as a function of the amplitude variations in said alternating voltage signal; and circuit means coupled between said plate and said control element for varying said control element in response to variations in said plate signal in a sense to oppose said variation in amplitude of said alternating voltage signal.

References Cited in the file of this patent UNITED STATES PATENTS 2,103,619 Hallmark Dec. 29, 1937 2,262,149 Slonczewski Nov. 11, 1941 2,512,658 Levy June 27, 1950 2,761,974 Burk et al Sept. 4, 1956 2,769,093 Hare et a1. Oct. 30, 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,847,574 August 12, 1958 Walter Donner It is herebf; certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 8, line 9, for "capacitor", first occurrence, read capacitance Signed and sealed this 4th day of November I958,

SEAL) ttest:

KARL I-I. AXLINE ROBERT C. WATSON 

